1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar electrophotographic image forming apparatus. More particularly, the present invention relates to a magnetic brush type developing method using a two-ingredient type developer made up of toner grains and carrier grains, and a developing device using the same.
2. Description of the Background Art
It is a common practice with an electrophotographic imager forming apparatus to form a latent image on an image carrier, which has a photoconductive layer on its surface, and develop the latent image with a developing device facing the image carrier to thereby produce a corresponding toner image. The developing device stores a developer that is often implemented as a two-ingredient type developer made up of toner grains and magnetic carrier grains and feasible for color image formation. This type of developer is frictionally charged by an agitator disposed in the developing device, so that the toner grains electrostatically deposit on the carrier grains. The carrier grains with the toner grains are conveyed on a sleeve or developer carrier, which is in rotation, by being magnetically retained on the sleeve by magnets arranged inside the sleeve.
One of the magnets disposed in the sleeve is a main magnet for development located at a position where the sleeve and image carrier are closest to each other. When the developer on the sleeve in rotation approaches the main magnet, the carrier grains of the developer gather and rise along the magnetic lines of force of the main magnet, forming a magnet brush having a number of brush chains.
As for a developing system of the type using the magnet brush, the carrier grains, which are dielectric, are considered to increase field strength between the image carrier and the sleeve and allow the toner grains to migrate from the tips of the brush chains toward the image carrier. In this type of developing system, the toner grains in the portion where the magnet brush formed by the carrier grains is absent has been used for development little. It has therefore been extremely difficult to increase the amount of toner to contribute to development in relation to the adjustment of the other conditions.
To implement high image density despite the limited region where the toner grains are usable, Japanese Patent No. 2,668,781, for example, discloses a developing method that uses an alternating electric field in order to use both of toner grains deposited on brush chains, which are formed by magnetic grains, and toner grains deposited on a developer carrier. This kind of scheme, however, cannot use toner grains other than toner grains deposited on the brush chains and developer carrier in a developing zone where the magnetic grains rub against an image carrier. It is therefore difficult to implement sufficient image quality. Further, the number of brush chains available with the magnetic grains is too small to realize, based on an electrode effect, a high-quality image with a smooth solid portion.
To enhance the contribution of toner grains to development, it is necessary to increase the ratio in which toner grains are used in a developing region. In practice, however, field strength varies in a complicated way in relation to a gap for development, which is the shortest distance between an image carrier and a sleeve, and the curvature of the image carrier, often failing to cause the toner grains to fly toward the image carrier. It is therefore extremely difficult to optimize the positional relation between the image carrier and the sleeve. This is particularly true when the image carrier has a small diameter, because a space between the image carrier and the sleeve sharply decreases toward the gap for development.
A current trend in the electrophotographic imaging art is toward color image formation as distinguished from monochromatic image formation. Generally, to fix a toner image formed by dry toner grains on a sheet or recording medium, use is made of a contact type, thermal fixing method that heats a roller, belt or similar fixing member. This type of fixing method has thermal efficiency high enough to implement high-speed fixation and can provide color toner with gloss and transparency. However, the problem with such a fixing scheme is that when the toner grains are pressed against the fixing member in a melted state and then peeled off, part of the toner image is transferred to the fixing member and then to another toner image to follow. This is generally referred to as toner offset.
To obviate toner offset, it has been customary to use, e.g., a fixing roller formed of silicone rubber or fluorocarbon resin desirable in parting ability, and to coat the surface of the roller with silicone oil or similar parting oil. Although the parting oil is successful to obviate toner offset, it must be accompanied by a coating device that increases the size and cost of a fixing device.
In light of the above, as for monochromatic toner, it has been proposed to adjust, e.g., the molecular weight distribution of binder resin in such a manner as to increase viscoelasticity when the toner melts, thereby preventing the melted toner from breaking. It has also been proposed to add wax or similar parting agent to the toner. These proposals are directed to the omission or the minimization of the parting oil to be coated on the fixing roller.
The parting agent added to the toner is undesirable in that it increases the adhesion of the toner and thereby obstructs the transfer of the toner to a sheet. Moreover, the parting agent contained in the toner smears a carrier or similar frictional charging member and thereby degrades the charging ability of the charging member, lowering durability.
Japanese Patent Laid-Open Publication No.8-220808, for example, teaches toner formed of linear polyester resin with a softening point of 90° C. to 120° C. and carnauba wax. Japanese Patent Laid-Open Publication No. 9-106105 teaches toner formed of resin and wax soluble in each other, but different in softening point. Japanese Patent Laid-Open Publication No. 9-304964 teaches toner defining the melting viscosity of polyester resin and that of wax specifically. Japanese Patent Laid-Open Publication No. 10-293425 teaches toner containing polyester resin with a softening point of 90° C. to 120° C. and rice wax, carnauba wax and silicone oil. Further, Japanese Patent Laid-Open Publication No. 5-612242 teaches polymerized toner containing wax.
The conventional magnet brush type developing method cannot easily implement sufficient image density or a smooth solid portion, as stated earlier. In addition, the toner containing a parting agent relatively easily coheres and is therefore low in developing ability and dot reproducibility. It is therefore difficult to realize high-quality images with this kind of toner.
More specifically, wax appears on the surfaces of toner grains containing a parting agent in order to obviate offset, aggravating cohesion of the toner grains. Generally, additives are added to toner in consideration of electrification, image transfer, toner scattering, toner spent and so forth. However, the expected effect is difficult to achieve with the toner containing a parting agent for a given amount of additives.
If a parting oil coating device can be omitted from the fixing device, then there can be realized cost reduction and simple construction. However, it is difficult to achieve high image quality with the conventional magnet brush type developing device, which uses toner containing a parting agent, because the parting agent lowers the developing ability and dot reproducibility.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent No. 2,829,927, Japanese Patent Publication No. 7-117769, and Japanese Patent Laid-Open Publication Nos. 5-289522, 7-128981, 10-73996, 2000-321814, 2001-324874, 2002-258618, 2002-278263, and 2002-278264.